Optical sheet, polarizer and liquid-crystal display device

ABSTRACT

An optical sheet constituted by a retardation film; and a transparent layer provided on one of opposite surfaces of the retardation film. The retardation film exhibits Nz=(nx=nz)/(nx=ny) in a range of from 0.1 to 0.4 and (nx−ny)d in a range of from 200 to 350 nm in which d is a thickness of the retardation film, nz is a refractive index in a direction of a Z axis expressing a direction of the thickness d of the retardation film, nx is a refractive index in a direction of an X axis expressing a direction of the retardation film in a plane perpendicular to the Z axis while the X axis also expresses a direction of the highest in-plane refractive index, and ny is a refractive index in a direction of a Y axis expressing a direction of the retardation film perpendicular both to the is Z axis and to the X axis. The transparent layer has a thickness not larger than 10 μm and exhibits refractive index anisotropy of nx∓ny&gt;nz.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical sheet by which lightbetween polarizers disposed in the form of crossed-Nicol can be cut offat wide-ranging azimuth angles so that a good-display-quality verticallyoriented liquid-crystal display device which is excellent in viewingangle and contrast can be formed.

[0003] The present application is based on Japanese Patent ApplicationNo. 2000-277596, which is incorporated herein by reference.

[0004] 2. Description of the Related Art

[0005] Even in the case where light between polarizers disposed in theform of crossed-Nicol was cut off in a normal-line (frontal) directionin which an axis of transmission and an axis of absorption functionednormally, there was heretofore a problem that light leakage occurred ina displaced viewing azimuth crossing the optical axis so that theintensity of leakage light increased gradually as the angle of obliqueviewing increased. The problem was revealed as a disadvantage thatdisplay quality was deteriorated by reduction in display contrast due tolight leakage in a viewing azimuth displaced from the optical axis whenthe polarizers were disposed on opposite sides of the liquid-crystalcell to function as a polarizer and an analyzer to thereby form aliquid-crystal display device.

[0006] In a vertically oriented (VA) liquid-crystal cell in whichliquid-crystal molecules were oriented approximately vertically withrespect to a cell substrate so that light was able to be transmittedwith little change of the plane of polarization, when polarizers weredisposed in the form of crossed-Nicol on opposite sides of the cell,light was apt to be cut of f in a frontal (normal) direction of adisplay panel perpendicular to the cell substrate in an inoperative modehaving no external voltage applied to thereby take it possible to formgood black display easily, in comparison with a TN liquid-crystal cellor the like in which liquid-crystal molecules were oriented horizontallywith respect to a cell substrate so that display quality was apt to bedeteriorated by light leakage owing to birefringence of theliquid-crystal cell at light transmission. However, the problem thatcontrast was deteriorated by light leakage in an oblique viewing azimuthdisplaced from the optical axis of the polarizers because of theaforementioned problem caused by the polarizers still remained, even ifa phase retarder exhibiting refractive index anisotropy of nx=ny>nz asdisclosed in Unexamined Japanese Patent Publication No. Sho. 62-210423was used in combination with the vertically oriented s liquid-crystalcell for compensating for birefringence of the liquid-crystal cellcaused by oblique viewing.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to develop a member capableof suppressing light leakage even in an oblique viewing azimuthdisplaced from the optical axis between polarizers disposed in the formof crossed-Nicol to thereby obtain a vertically oriented liquid-crystaldisplay device which is good is in display quality and which isexcellent in contrast at wide-ranging viewing angles.

[0008] According to an aspect of the present invention, there isprovided an optical sheet constituted by a retardation film, and atransparent layer provided on one of opposite surfaces of theretardation film. The retardation film exhibits Nz=(nx−nz)/(nx−ny) in arange of from 0.1 to 0.4 and (nx−ny)d in a range of from 200 to 350 nmin which d is a thickness of the retardation film, nz is a refractiveindex in a direction of a Z axis expressing a direction of the thicknessd of the retardation film, nx is a refractive index in a direction of anX axis expressing a direction of the retardation film in a planeperpendicular to the Z axis while the X axis also expresses a directionof the highest in-plane refractive index, and ny is a refractive indexin a direction of a Y axis expressing a direction of the retardationfilm perpendicular both to the Z axis arid to the X axis. Thetransparent layer has a thickness not larger than 10 μm and exhibitsrefractive index anisotropy of nx≈ny>nz.

[0009] According to another aspect of the present invention, there isprovided a polarizer constituted by a laminate of an optical sheet asdefined above, and a polarizing film. According to a further aspect ofthe present invention, there is provided a liquid-crystal display deviceconstituted by a vertically oriented liquid-crystal cell, and a pair ofpolarizers as defined above, the pair of polarizers being provided onopposite sides of the cell. Further, a transparent layer in each of thepair of polarizers is positioned on corresponding one of opposite sidesof the cell, and the pair of polarizers provided on the opposite sidesof the cell are disposed in the for of crossed-Nicol.

[0010] When the optical sheet according to the present invention is usedas a transparent protective layer or the like, a polarizer can be formedon the optical sheet. When polarizers defined above are disposed in theform of crossed-Nicol on opposite sides of a vertically orientedliquid-crystal cell, light can be cut off in the direction of theoptical axis of the polarizers in the same manner as that in thebackground art while birefringence of the liquid-crystal cell in anoblique viewing direction can be compensated for. Hence, light leakagecan be suppressed even in an azimuth displaced from the optical axis.Accordingly, there can be formed a vertically oriented liquid-crystaldisplay device which is good in display quality and which is excellentin contrast at wide-ranging viewing angles. Because the optical sheetcan be used as a transparent layer for protecting a polarizing film,reduction in thickness of the polarizer or the liquid-crystal displaydevice can be attained.

[0011] Features and advantages of the invention will be evident from thefollowing detailed description of the preferred embodiments described inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0012]FIG. 1 is a sectional view showing an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The optical sheet according to the present invention isconstituted by a retardation film; and a transparent layer provided onone of opposite surfaces of the retardation film, wherein theretardation film exhibits Nz=(nx−nz)/(nx−ny) in a range of from 0.1 to0.4 and (nx−ny)d in a range of from 200 to 350 nm in which d is athickness of the retardation film nz is a refractive index in adirection of a Z axis expressing a direction of the thickness d of theretardation film, nx is a refractive index in a direction of an X axisexpressing a direction of the retardation film in a plane perpendicularto the Z axis while the X axis also expresses a direction of the highestin-plane refractive index, and ny is a refractive index in a directionof a Y axis expressing a direction of the retardation film perpendicularboth to the Z axis and to the X axis, and wherein the transparent layerhas a thickness not larger than 10 μm and exhibits refractive indexanisotropy of nx≈ny>nz.

[0014]FIG. 1 shows an example of the optical sheet. In FIG. 1, theoptical sheet 1 is constituted by a retardation film 11 (13), and atransparent layer 12 (14) Incidentally, FIG. 1 shows a liquid-crystaldisplay device which is constituted by a pair of optical sheets 1defined above, a pair of polarizing films 2 each including a transparentprotective layer 21, and a vertically oriented liquid-crystal cell 3.

[0015] As each of the retardation films, it is possible to use asuitable one which exhibits Nz≈(nx−nz)/(nx−ny) in a range of from 0.1 to0.4, preferably in a range of from 0.2 to 0.3, and a retardation(nx−ny)d in a range of from 200 to 350 nm, preferably in a range of from250 to 280 nm in which d is the thickness of the retardation film, nz isthe refractive index in a direction of a Z axis expressing a directionof the thickness d of the retardation film, nx is the refractive indexin a direction of an X axis expressing a direction of the retardationfilm in a plane perpendicular to the Z axis while the X axis alsoexpresses a direction of the highest in-plane refractive index, and nyis the refractive index in a direction of a Y axis expressing adirection of the retardation film perpendicular both to the Z axis andto the X axis.

[0016] Any suitable polymer may be used for forming the retardation filmwithout any particular limitation. Examples of the polymer may include:polycarbonate or polyallylate; polysulfone or polyolefin such aspolypropylene; polyester such as polyethylene terephthalate orpolyethylene naphthalate; norbornene polymer or acrylic polymer; styrenepolymer or cellulose polymer; a mixture polymer formed by selecting twokinds of polymers or three or more kinds of polymers from thesepolymers; and so on.

[0017] For example, the retardation film having the aforementionedcharacteristic can be obtained as a drawn film or the like constitutedby a polymer film drawn by a suitable system such as uniaxial drawing orbiaxial drawing. Preferably, the retardation film is a polymer film inwhich optical characteristic such as retardation can be controlled inaccordance with the change of the kind of polymer, the drawingcondition, and so on, and which is excellent in light transmittance andsmall in orientation irregularity and retardation irregularity. Theretardation film may be a suitable one which is made from a polymer filmprovided with a heat-shrinkable film bonded to the polymer film so thatthe thicknesswise refractive index of the polymer film is controlledunder the action of shrinking force of the heat-shrinkable film byheating. Alternatively, the retardation film may be provided as amultilayer film of two or more retardation layers in order to controloptical characteristic.

[0018] The transparent film provided on one of opposite surfaces of theretardation film has a thickness not -larger than 10 μm and exhibitsrefractive index anisotropy of nx≈ny>nz in which nz is the refractiveindex in a direction of a Z axis expressing a direction of the thicknessof the transparent film, nx is the refractive index in a direction of anX axis expressing a direction of the transparent film in a planeperpendicular to the Z axis, and ny is the refractive index in adirection of a Y axis expressing a direction of the transparent filmperpendicular both to the Z axis and to the X axis The relation nx≈nymeans that a variation of 10 nm or less on the basis of the retardationgiven by the product of |nx−ny| and the thickness of the transparentlayer can be allowed. Hence, the relation nx≈ny includes the case ofnx=ny.

[0019] When the thickness of the transparent layer is selected to be notlarger than 10 μm, reduction in thickness of the optical sheet can beachieved so that the layer can be used as a transparent layer forprotecting the polarizing film. The transparent layer can be formed froma suitable material exhibiting the aforementioned refractive indexanisotropy and by a suitable system A coating system using an organicmaterial is used preferably from the point of view of forming a flexiblethin layer easily, and so on. A suitable system such as gravure coating,die coating or dip coating can be used as the coating system. A systemof transferring a coating solution layer or a coating film provided onanother film may be also used.

[0020] The material preferably used for forming the transparent layerfrom the point of view of achieving the refractive index anisotropy ofnx≈ny>nz while satisfying the aforementioned reduction in film thicknessis a material which can form a cholesteric liquid-crystal layerconstituted by a cholesteric liquid-crystal polymer, achiral-agent-mixed nematic liquid-crystal polymer, a compound capable offorming such a liquid-crystal polymer by photopolymerization orthermalpolymerization, or the like. Especially, the material capable offorming a cholesteric liquid-crystal layer used preferably from thepoint of view of achieving bright display is a material which does notexhibit selective reflection characteristic in a visible light region.

[0021] That is, the cholesteric liquid-crystal layer exhibitscharacteristic of selectively reflecting a part of light withwavelengths near to the wavelength ncP of incident light in parallel toa helical axis as circularly polarized light in one of left and rightdirections with the wavelength ncP as its center wavelength when nc isthe average refractive index on the basis of the helically orientedstate of the cholesteric liquid-crystal layer, and P is the helicalpitch. Hence, if the region of the selectively reflected light appearsin a visible light region, the intensity of light allowed to be used fordisplay is reduced disadvantageously. Any suitable orientation systemsuch as a process of forming an oriented film by a rubbing treatment oran orientation process using application of electric field or magneticfield can be applied to the formation of the cholesteric liquid-crystallayer.

[0022] Incidentally, the thickness of the transparent layer is generallyselected to be not smaller than 0.1 μm, particularly not smaller than0.5 μm, more particularly not smaller than 1 μm. The refractive indexanisotropy of nx≈ny>nz in the transparent layer means the fact that nzis smaller than each of nx and ny. The refractive index difference isnot particularly limited and can be determined suitably in accordancewith the birefringent characteristic, or the like, of the verticallyoriented liquid-crystal cell to be compensated for.

[0023] Even in the case where the optical sheet according to the presentinvention is laminated on a background-art polarizer, the aforementionedeffect of preventing light leakage can be obtained. The polarizerpreferred from the point of view of handling properties due to reductionin thickness or increase inflexibility is, however, a polarizerconstituted by an optical sheet 1 defined above, and a polarizing film 2having a surface on which the optical sheet 1 is laminated as atransparent protective layer, as shown in FIG. 1. Particularly from thepoint of view of improvement in the effect of compensating for thevertically oriented liquid-crystal cell, the polarizing film 11 (13) islaminated on a side of the optical sheet 1 opposite to the transparentlayer 12 (14) side so that the X axis direction (the direction of thehighest in-plane refractive index) of the retardation film 11 (13)crosses the absorption axis thereof as perpendicularly as possible.

[0024] As the polarizing film, any suitable one in accordance with thebackground art may be used without particular limitation. That is, it ispossible to use a film of a hydrophilic polymer, such as polyvinylalcohol, partially formalized polyvinyl alcohol or partially saponifiedethylene-vinyl acetate copolymer, which is made to absorb a dichromaticmaterial containing iodine or/and a dichromatic dye such as an azo dye,an anthraquinone dye or a tetrazine dye so that the resulting film isdrawn and oriented.

[0025] The optical sheet and the polarizing film may be formed to beseparated from each other. It is, however, preferable from the point ofview of preventing displacement of the optical axis and preventingentrance of foreign matter such as dust that the optical sheet and thepolarizing film are fixed to each other. A suitable system such as abonding system using a transparent adhesive layer can be applied to thefixation (lamination). The adhesive agent or the like used in thebonding system is not particularly limited in kind. An adhesive agentwhich does not require any high-temperature process for hardening anddrying the adhesive agent in the bonding process is used preferably fromthe point of view of preventing the optical characteristic of theconstituent members from changing. That is, an adhesive agent whichrequires neither long-term hardening process nor long drying time isused preferably. From this point of view, a hydrophilic polymer adhesiveagent or an adhesive layer may be used preferably.

[0026] A transparent adhesive agent made of a suitable polymer such asacrylic polymer, silicon polymer, polyester, polyurethane, polyether orsynthetic rubber can be used for forming the aforementioned adhesivelayer. Particularly, an acrylic adhesive agent is used preferably fromthe point of view of optical transparency, adhesive characteristic,weather resistance, etc. As for the adhesive layer, one adhesive layermay be provided on one of opposite surfaces of the polarizer, or twoadhesive layers may be provided on the opposite surfaces of thepolarizer, for the purpose of bonding the liquid-crystal cell to asubject to be bonded in accordance with the necessity. in this case, itis preferable that a separator or the like is temporarily attached tothe adhesive layer to prevent the surface of the adhesive layer fromcontamination or the like before the adhesive layer is exposed to theoutside for practical use.

[0027] When the surface of the polarizing film on a side opposite to theoptical sheet side in the polarizer is exposed, a transparent protectivefilm may be provided on the surface for the purpose of reinforcement,improvement of heat resistance and moisture resistance, etc. Thetransparent protective layer can be formed in accordance with thebackground art. That is, the transparent protective layer can be formedas a coating layer of a suitable resin such as triacetyl cellulose or asa layer of a laminate of resin films. The transparent protective layermay contain fine particles for the purpose of diffusing light,roughening the surface, etc.

[0028] The polarizer may be formed as a plate having a suitablefunctional layer such as a protective layer used for various purposes ofwater resistance, and so on, or an anti-reflection layer or/and ananti-glare layer used for the purpose of preventing surface reflection,in one or each of opposite surfaces of the polarizer, in the same manneras described above in the transparent protective layer. Theanti-reflection layer can be formed suitably as a light-coherent filmsuch as a fluorine polymer coat layer or a multilayer metal depositedfilm. The anti-glare layer can be formed by a suitable system in whichsurface-reflected light is diffused by a coating layer of a resincontaining fine particles or by a fine asperity structure given to thesurface by a suitable system such as embossing, sandblasting or etching.

[0029] Examples of the fine particles include inorganic particles andorganic particles with a mean particle size of from 0.5 to 20 μm. Theinorganic particles which may be electrically conductive are made ofsilica, calcium oxide, alumina, titania, zirconia, tin oxide, indiumoxide, cadmium oxide, antimony oxide, etc. The organic particles aremade of crosslinked or non-crosslinked suitable polymers such aspolymethyl methacrylate and polyurethane. One member or a combination oftwo or more members suitably selected from the inorganic particles andthe organic particles may be used as the fine particles.

[0030] The optical sheet or polarizer according to the present inventioncan be used preferably for suitable purposes such as the purpose offorming a liquid-crystal display device. Particularly, the polarizer, inwhich a polarizing film is laminated on the surface of the optical sheetopposite to the transparent layer side so that the direction of thehighest in-plane refractive index of the retardation film crosses theabsorption axis thereof as perpendicularly as possible, can be usedpreferably for optical compensation for the vertically orientedliquid-crystal cell. The liquid-crystal display device can be formed byarranging polarizers in the form of crossed-Nicol on opposite sides ofthe vertically oriented liquid-crystal cell 3 while positioning thetransparent layers 12 and 14 of the polarizers on the opposite sides ofthe liquid-crystal cell.

[0031] The polarizers preferred from the point of view of achievingwide-ranging viewing angles by the high-grade compensation arepolarizers which are disposed on both sides of the liquid-crystal celland in which the sum of the absolute values of the thicknesswiseretardations of the transparent layers in the polarizers is in a rangeof from 0.5 time to 1.3 times, particularly in a range of from 0.7 timeto 1.0 time as large as the absolute value of the thicknesswiseretardation of the liquid-crystal cell. Incidentally, the thicknesswiseretardation in each of the transparent layers is defined by the productof {(nx+ny)/2−nz} and the thickness of the transparent layer.

[0032] In the present invention, the liquid-crystal display device canbe formed in accordance with the background art without any particularlimitation except that the aforementioned polarizers are disposed in theform of crossed-Nicol on opposite sides of the vertically-orientedliquid-crystal cell. Hence, a system for operating the liquid-crystalcell is not particularly limited. When the liquid-crystal display deviceis to be formed, suitable optical devices such as a phase retarder, alight-diffusing plate, a backlight, a light-condensing sheet, areflection plate, etc. may be disposed suitably in accordance with thenecessity.

Example 1

[0033] A rubbed and oriented film was formed on one of opposite surfacesof a retardation film constituted by a drawn film of polycarbonate andexhibiting Nz of 0.25 and (nx−ny)d of 260 nm. A cholesteric liquidcrystal (CB-15 made by Dai Nippon Ink and Chemicals, Inc.) was appliedonto the rubbed and oriented film and dried so as to form a transparentfilm constituted by a coating film which had a thickness of 5 μm andwhich exhibited refractive index anisotropy of nx≈nv>nz. Thus, anoptical sheet was obtained.

[0034] On the other hand, a polyvinyl alcohol film was dyed in anaqueous solution containing iodine and then uniaxially drawn betweendifferent-circumferential-velocity rolls in an aqueous solutioncontaining boric acid to thereby obtain a polarizing film. The drawnfilm-exposed side of the optical sheet was bonded onto one of oppositesurfaces of the polarizing film through a polyvinyl alcohol adhesiveagent. A triacetyl cellulose film was bonded onto the other surface ofthe polarizing film through a polyvinyl alcohol adhesive agent. Thus, apolarizer was obtained. Incidentally, the optical sheet was laminated onthe polarizing film so that the direction (nx) of the highest in-planerefractive index of the drawn film in the optical sheet crossed the axisof absorption of the polarizing film perpendicularly.

[0035] Polarizers as defined above were disposed in the form ofcrossed-Nicol on opposite sides of a vertically oriented liquid-crystalcell while the optical sheet in each of the polarizers was disposed onthe cell side. Thus, a liquid-crystal display device was formed. In thiscase, the sum of the absolute values of the thicknesswise retardationsof the transparent layers in the polarizers disposed on the oppositesides of the liquid-crystal cell was 0.8 times as large as the absolutevalue of the thicknesswise retardation of the liquid-crystal cell. Thecontrast ratio was measured in the condition that the obtainedliquid-crystal display device was viewed obliquely at an inclinationangle of 70 degrees with respect to the normal line in an azimuthdisplaced by 45 degrees from the optical axis of the polarizers disposedin the form of crossed-Nicol (this condition applies to the followingComparative Examples). As a result ( the contrast ratio was 12.

Comparative Example 1

[0036] A polarizer was obtained in the same manner as that in Example 1except that the optical sheet was replaced by a triacetyl cellulosefilm. Polarizers obtained thus were used to form a liquid-crystaldisplay device. The contrast ratio in the predetermined oblique viewingdirection was 2.

Comparative Example 2

[0037] A liquid-crystal display device was formed in the same manner asthat in Comparative Example 1 except that a retardation film which wasconstituted by a drawn film of polycarbonate exhibiting refractive indexanisotropy of nx≈ny>nz and in which the sum of the absolute values ofthe thicknesswise retardations was 0.8 times as large as the absolutevalue of the thicknesswise retardation of the liquid-crystal cell wasinterposed between one of the polarizers and the liquid-crystal cell tothereby be a compensator for the liquid-crystal display device. Thecontrast ratio in the predetermined oblique viewing direction was 6.

[0038] Although the invention has been described in its preferred formwith a certain degree of particularity, it is understood that thepresent disclosure of the preferred form can be changed in the detailsof construction and in the combination and arrangement of parts withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:
 1. An optical sheet comprising: a retardation film;and a transparent layer provided on one of opposite surfaces of saidretardation film; wherein said retardation film exhibitsNz=(nx−nz)/(nx−ny) in a range of from 0.1 to 0.4 and (nx−ny)d in a rangeof from 200 to 350 nm in which d is a thickness of said retardationfilm, nz is a refractive index in a direction of a Z axis expressing adirection of the thickness d of said retardation film, nx is arefractive index in a direction of an X axis expressing a direction ofsaid retardation film in a plane perpendicular to said Z axis while saidX axis also expresses a direction of the highest in-plane refractiveindex, and ny is a refractive index in a direction of a Y axisexpressing a direction of said retardation film perpendicular both tosaid Z axis and to said X axis; and wherein said transparent layer has athickness not larger than 10 μm and exhibits refractive index anisotropyof nx≈ny>nz.
 2. An optical sheet according to claim 1, wherein saidtransparent layer is made of a coating film of an organic material. 3.An optical sheet according to claim 1, wherein said transparent layer isconstituted by a cholesteric liquid-crystal layer.
 4. A polarizercomprising a laminate of an optical sheet according to claim 1 and apolarizing film.
 5. A polarizer according to claim 4, wherein saidpolarizing film is disposed on a side of said optical sheet opposite tothe transparent layer side of said optical sheet so that said x axisdirection of said retardation film in said optical sheet crosses an axisof absorption of said retardation film perpendicularly.
 6. Aliquid-crystal display device comprising a vertically orientedliquid-crystal cell; and a pair of polarizers each according to claim 5,said pair of polarizers being provided on opposite sides of said cell;wherein a transparent layer in each of said pair of polarizers ispositioned on corresponding one of opposite sides of said cell; andwherein said pair of polarizers provided on said opposite sides of saidcell are disposed in the form of crossed-Nicol.
 7. A liquid-crystaldisplay device according to claim 6, wherein a sum of absolute values ofthicknesswise retardations each defined by a product of {(nx+ny)/2−nz}and a layer thickness of said transparent layer in each of said pair ofpolarizers disposed on the opposite sides of said liquid-crystal cell isin a range of from 0.5 times to 1.3 times as large as an absolute valueof a thicknesswise retardation of said liquid-crystal cell.